Designing a Lithium-Ion Cell for Studies of a Single Degradation Mechanism Over a Wide Temperature Range

Abstract

18650-sized cylindrical cells containing single crystal Li[Ni 0.6 Mn 0.4 Co 0.0 ]O 2 , Li[Ni 0.6 Mn 0.35 Co 0.05 ]O 2 , and Li[Ni 0.6 Mn 0.3 Co 0.1 ]O 2 positive electrodes along with artificial graphite negative electrodes were constructed to be balanced at 4.05 V. These cells were designed so that they would have only the single degradation mode of lithium inventory loss due to the solid-electrolyte interphase layer growth. Cells were cycled both at C/3 and C/20 over a wide temperature range from 20 to 100 °C in order to accelerate degradation processes at higher temperatures and more rapidly predict low-temperature behaviour. A low upper cutoff voltage of 4.0 V was selected to avoid electrolyte oxidation, and an electrolyte composition incorporating pure lithium bis(fluorosulfonyl)imide salt was chosen based on the temperature and voltage range of operation. A thorough post-cycling analysis was performed to verify the elimination of all degradation modes except inventory loss and minor impedance growth, which enabled the application of a simple square root time model to make accurate lifetime predictions. In addition, the capacity retention of these cells at elevated temperature is incredible, with the best cells retaining 87% capacity after 1400 C/3 cycles (one year) continuously at 85 °C.